Such flaps are used in installations for heating and ventilating or air conditioning the cabins of motor vehicles. The flap can be moved between two extreme positions, eg. by means of a cable guided along a sheath and by a control lever which is movable across the vehicle dash board.
The force that needs to be applied to the control lever and hence to the cable in order to move the lever and the flap must always be greater than the sum of the other forces acting on the flap. Said other forces include interference forces due to vibration and the weight of the flap, pressure forces due to the flow of air near the flap, the force required to hold the flap in the position set by the lever on the dash board, and friction forces between the cable and its sheath.
The force that needs to be applied to move the control lever is therefore quite substantial, and it is desirable at present to reduce this force so that the control becomes easier and more agreeable to operate and also more accurate.
It has also been observed that, under certain conditions, the interference forces on the flap can exceed the forces holding the flap in the position set by the control lever, in which case the flap moves to a position which corresponds to conditions of heating, ventilating, or air conditioning in the vehicle cabin other than that selected by the user.
Preferred embodiments of the invention avoid this drawback by flap-retaining means which are simple, effective, and very cheap, and which do not go against vehicle manufacturers' requirements for a control lever which is easy to move.